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Astronomers kill all the fun, blame dust for Betelgeuses dimming

Enlarge / This image is a colour composite made from exposures from the Digitized Sky Survey 2 (DSS2). It shows the area around the red supergiant star Betelgeuse.ESO/Digitized Sky Survey 2/Davide De Martin.

Betelgeuse is one of the closest massive stars to Earth. It's also an old star, and it has reached the stage when it glows a dull red and expands, with the hot core only having a tenuous gravitational grip on its outer layers. This combination means that we're actually able to resolve different areas on the star's surface, despite the fact that it resides over 700 light years away.

That ability came in handy late last year when Betelgeuse did something unusual: it dimmed so much that the difference was visible to the naked eye. Telescopes pointed at the giant were able to determine that—rather than a tidy, uniform drop in luminance—Betelgeuse's dimming was unevenly distributed, giving the star an odd, squished shape when viewed from Earth. That raised lots of questions about what was going on with the giant, with some experts speculating that, because of Betelgeuse's size and advanced age, the strange behavior was a sign of a supernova in the making.

Now, an international team of international observers is here to throw cold water on the possible explosion. Said researchers were lucky to have the Hubble pointed at Betelgeuse before, during, and after the dimming event. Combined with some timely ground observations, this data indicates a rather mundane reason for the star getting darker: a big burp that formed a cloud of dust near the star.

Stellar indigestion

There's a couple of things going on at the same time with Betelgeuse. One is that the star has something akin to a heartbeat, albeit an extremely slow and irregular one. Over time, the star cycles through periods when its surface expands and then contracts. One of these cycles is fairly regular, taking a bit over five years to complete. Layered on that is a shorter, more irregular cycle that takes anywhere from under a year to 1.5 years to complete.

We can track these cycles because the surface of the star moves toward or away from us, depending on whether it's in the expansion or contraction phases. This motion imparts a Doppler shift to the light coming to us from Betelgeuse's surface, and it causes some features in the star's light to shift to the red or blue end of the spectrum depending on whether it's contracting or expanding, respectively.

While they're easy to track with ground-based telescopes, these shifts don't cause the sort of radical changes in the star's light that would account for the changes seen during the dimming event.

But these shifts aren't the only things going on with Betelgeuse. As we mentioned earlier, the star is a huge, bloated structure, with all the convection and magnetic field activity that we see driving behavior on other stars. These can drive unusual behavior, and that's just the sort of thing that Hubble was being used to track. The researchers behind the new work had gotten time on the telescope to do observations in the UV area of the spectrum, which struggles to get through Earth's atmosphere.

These UV observations showed that the photosphere—an outer layer of the star that produces the light we see—started accelerating outward in advance of the dimming event. By the time this acceleration reached its peak, the photosphere was moving at about seven kilometers a second. Critically, this peak came somewhat before the dimming, and the outward push had reversed by the time the dimming became dramatic. Also notable was the fact that the indications of this acceleration weren't evenly distributed across the star; only part of the star's surface expRead More – Source

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